Plasma sensors and related methods

ABSTRACT

Plasma sensors, systems and related methods are described. An example method for predicting an event includes providing a carrier signal across two electrodes and forming a plasma between the two electrodes. The example method also includes measuring a modulated signal from the plasma, manipulating the modulated signal to produce a value and comparing the value to a threshold. Finally, the example method includes determining the likelihood of the event based on the comparison.

CROSS REFERENCE TO RELATED APPLICATION

This application is a non-provisional application claiming priority fromU.S. Provisional Application Ser. No. 60/998,219, filed Oct. 9, 2007,entitled “Plasma Sensor for Stall Precursor Detection in Gas-TurbineEngines” and incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to measurement systems and,more particularly, to plasma sensors and related methods of use.

BACKGROUND OF RELATED ART

The safety and efficiency of axial flow fans and compressors, such as,for instance, gas turbine engines are typically limited, in part, by theperformance of the compressors that supply high pressure air forcombustion. Gas turbine engines are subject to rotating stall in thecompressor section during operation, which is of particular concern toaircraft designers. Stall has a negative impact on the overallperformance of the engine including the ability to attain maximum fuelefficiency. As there are no known diagnostic sensors available to theengine operator to predict incipient stall, the engine must be operatedwith fuel management strategies that maintain the compressor at safestall margins over a wide range of operating pressures and speeds.Operating the engine at these conservative settings typically preventthe engine from attaining maximum performance and efficiency which canusually only be obtained near conditions of stall at reduced stallmargins.

Active management of the incipient stall process would make it possibleto reduce the stall margin during operation. However, prior techniquesthat attempt to successfully manage the stability of the compressorsection have used sensors (e.g., Kulite® sensors) that have provedincapable of surviving the harsh operating environments typical offull-scale compressors (e.g., the operating conditions found injet-engines). In particular, heat and vibration typically contribute toan almost instantaneous destruction of the sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example plasma sensor system.

FIG. 2 illustrates a more detailed diagram of a plasma probe portion ofthe example plasma sensor system of FIG. 1.

FIG. 3 illustrates a more detailed diagram of an example electrodeincluded the example plasma probe portion of FIG. 2.

FIGS. 4A-4B illustrate enlarged views of the example electrode of FIG.3, included in the example plasma sensor system of FIG. 1.

FIG. 5A shows a portion of the example plasma sensor of FIG. 1 embeddedin an example casing.

FIG. 5B is a view similar to FIG. 5A showing an example rotor andexample carrier and modulated signals.

FIGS. 6A-6D illustrate example plasma discharges between two electrodesin the presence of different example flow disturbances.

FIG. 7 shows a demodulation of the example modulated signal of FIG. 5B.

FIG. 8 is an example interface showing example processed signals.

FIG. 9 is an enlarged view of a portion of the example plasma sensorsystem of FIG. 1 showing an example rotor and casing.

FIG. 10 is an example voltage spectrum plot showing an exampleblade-passage frequency.

FIG. 11 is a plot of an example blade passage frequency.

FIG. 12 is an example voltage plot showing an example stall.

FIG. 13 shows an example voltage plot and example correlation indexshowing an example stall.

FIG. 14 shows an example voltage plot and example correlation indexshowing example rotating stall cells.

DETAILED DESCRIPTION

The following description of the disclosed examples is not intended tolimit the scope of the disclosure to the precise form or forms detailedherein. Instead the following description is intended to be illustrativeof the principles of the disclosure so that others may follow itsteachings. It will be appreciated by one of ordinary skill in the artthat while the disclosed examples are directed to a compressor casingfor a gas turbine engine, the disclosed sensors may be utilized toprovide stall or surge warnings for any suitable axial flow device,including, but not limited to, fans, turbines, pumps, jet engines, highspeed ship engines, power stations, superchargers, low pressurecompressors, high pressure compressors, and/or any other application. Inaddition, as used herein, the term “sensor” may refer to a plasma sensorsystem, plasma sensor, plasma probe, plasma anemometer, etc. Also,“plasma” and “glow discharge” are used interchangeably throughout thisdisclosure.

Compressor sections of gas-turbines are subject to stall when themass-flow through the system falls below a critical value. Precursors tocompressor stall may exist in the form of transient short-wavelengthdisturbances in the unsteady pressure in the tip-gap region of thecompressor rotor. These precursors contribute to a reduction in theper-rotation coherence of a blade pressure signal, which can be anindicator of incipient stall. A warning of stall can be obtained duringoperation by performing real-time statistical auto-correlationmeasurements of the blade-passing pressure signal near the mid-chordlocation of the rotor.

Active management of the incipient stall process would make it possibleto reduce the stall margin during operation. The stall margin can bereduced by analyzing the time-resolved pressure history of a compressorsection, which, as detailed below, utilizes a real-time auto-correlationof the time-resolved rotor-tip pressure signature. The per-bladecoherence of the blade-passage pressure signature is monitored tocompute a correlation index whose value varies in relation to theloading on the rotor blades. During operation conditions in which thecompressor is lightly loaded, the coherence in the pressure signal ishigh and near unity. However, as the loading on the compressor blades isincreased the pressure signature of the blade passing becomes morechaotic, which reduces the coherence measure when comparing onerevolution to the next. This is caused by random disturbances in thepressure time series that increase as the compressor approaches stall.With the selection of an appropriate threshold in the correlation, it ispossible to predict a stall before it occurs. With a stall predicted, itis possible to employ an active stability management process to enablean operator to take the necessary steps to avoid a stall.

The examples described herein are generally directed to an alternatingcurrent (AC) driven, plasma sensor (e.g., anemometer) for measuring flowdisturbances at, for example, hypersonic Mach numbers. Flow disturbancesmay be, for example, pressure changes sensed adjacent to a flow path,velocity changes or any other disruption or change sensed in or from aflow. As provided in greater detail below, the example sensors describedherein utilize an AC driven glow discharge or plasma discharge createdin a small spatial volume between two electrodes that may beencapsulated as the primary sensing element. The plasma discharge ispreferably driven by an AC power source, such as a low power (e.g., lessthan about 5 Watt) AC source. The plasma created at the electrodesinteracts with disturbances in the flow that pass over or inside the gapbetween the electrodes, which can be detected by monitoring thetime-resolved voltage drop across the two electrodes, this voltagechange affects the amplitude of the AC carrier.

This time-resolved or time-varying, if unsteady, voltage drop ormodulation can be correlated to the unsteady velocity, pressure ormass-flux characteristics in the external flow, depending on orientationof the sensor. The change in plasma voltage as the flow disturbance inthe gap varies can be explained by the effect of the flow disturbance onthe current-carrying particles in the electrode gap. The ionized andmeta-stable species involved in the glow discharge experience a dragforce in the direction of the flow disturbance. This drag willcollectively deflect the trajectory of the plasma particles towards thedownstream edge of the electrodes as they traverse the gap. Some ofthese particles may be swept out of the gap entirely, at which pointthey no longer contribute to the current flow in the device. This isregistered as a rise in voltage. The frequency response of the sensor isquite high (for example 15 MHz) and is determined by the ion mobility ofthe plasma in the gap. The frequency bandwidth is set by the AC carrierdriving the plasma discharge, as modulation of a carrier cannot occur ata frequency higher than the carrier. In other words, as the glowdischarge may include ionized gas particles that have high mobility, theeffective mass of the sensing element in the sensor is quite low, whichenables a frequency response in excess of, for example, about 15 MHz andis effectively limited only by the frequency of the AC waveform used tocreate the plasma. Also, as there are no moving parts, these examplesensors feature a high level of mechanical robustness.

In addition, some examples of the plasma sensor include one or more ofthe following additional advantages: the example sensor requires nofrequency compensation up to its AC carrier frequency, the examplesensor has an amplitude-modulated output that has excellent common-moderejection with a signal-to-noise ratio that is improved over the outputgenerated by hot-wire devices, and the example sensor does not include asensor element that could easily break, the example sensor may have asmall spatial volume, the example sensor is insensitive to temperaturevariations (i.e., temperature independent) making it easier to calibratethan thermal-based sensors, and the example sensor may be operatedacross a myriad of different pressures (e.g., from very low pressuressuch as in a vacuum to very high pressures). With respect to thetemperature, in some examples, the sensors are impervious to hightemperature up to the melting point of the electrode materials used,which if Iridium-based electrodes are used, is higher than 1800° C. Theexample plasma sensor(s) described herein can survive the vibration andtemperatures of a full-scale compressor while providing the bandwidthnecessary to resolve the blade passage signature required by thecoherence technique described herein and while providing wirelesscapability, as detailed below. In addition, the example sensorsdescribed herein may have bandwidths in excess of 1 MHz for high-speed,high-enthalpy flows and do not require the use of external frequencycompensation circuitry. Also, the example sensor, or plasma anemometerprovides small spatial volume point measurements of velocity or pressurefluctuations with a frequency response that is unmatched by traditionalsensors.

Further, the example plasma sensors may be used in a variety ofapplications and environments such as, for example, for measurements ina turbine (e.g., gas-turbine machinery), shock tubes, shock-boundarylayer experiments, high-enthalpy hypersonic flows, in plasma tunnels,etc.

The example plasma sensor is successful in measuring important featuresof compressor stall. This includes the blade passage unsteadiness, thereduction in correlation that occurs before stall, the appearance ofrotating stall cells as well as the full stall event in a transonicaxial compressor. The unsteady voltage characteristics compare favorablywith respect to the dynamics of the compressor stall, as discussedbelow.

The example plasma sensor also has the ability to transmit the voltagesignal wirelessly, as mentioned above. This capability is due to thefact that the sensor is driven with a high-voltage AC waveform. Thiswaveform naturally broadcasts electromagnetic energy which can becaptured with a suitably designed antenna. This offers the possibilityof implementing an in-situ measurement device that could be used in agas-turbine compressor for stability management without the need to feedwires out of the engine; the signal could be broadcast wirelessly to anAM antenna mounted on the compressor casing.

The example sensors provide the path to implementations of stallprediction and stability management on full scale flight sizedcompressor sections typical to those used in aero jet-engines. Theexample sensors can survive the mechanical and thermal stressesencountered in a full size gas-turbine engine while providing thebandwidth necessary to resolve the transient pressure signature that canprovide prediction of compressor stall. This predictive warning enablesthe implementation of a stability management system that allows thecompressor to be operated much closer to the stall margin, thusincreasing the operating efficiency of the compressor.

One example method described herein includes a method for predicting anevent including providing a carrier signal across two electrodes andforming a plasma between the two electrodes. The example method alsoincludes measuring a modulated signal from the plasma, manipulating themodulated signal to produce a value and comparing the value to athreshold. Finally, the example method includes determining thelikelihood of the event based on the comparison.

An example plasma sensor system described herein includes a carriersignal generator, two electrodes and a plasma between the two electrodesformed by applying the carrier signal across the two electrodes. Theexample system also includes a probe to measure a modulated signal fromthe plasma and a processor to demodulate the sensor output to produce asignal that can be used in an auto-correlation scheme that gives apre-cursor of an imminent event. In the example described herein, theevent is a stall event. However, in other examples, the event may be asurge, flow oscillation, flow reversal, etc.

Furthermore, an example plasma sensor described herein includes meansfor capturing evidence of a flow disturbance across two electrodes andmeans for manipulating the evidence to produce a value indicative of theincipience of an event such as, for example, compressor stall.

FIG. 1 illustrates an example plasma sensor and system 100, which, inthis example is shown as an alternating current (AC) plasma anemometer.However, other examples may include other types of sensors and/or adirect current (DC). The example plasma sensor includes a signalgenerator 102, a transformer 104, a cable 106, a plasma probe 108, avoltage probe 110, a signal measurement device 112, and a computer 114.The signal generator 102 may be any type of suitable system forgenerating a periodic voltage or current signal, such as, for example,an AC signal. In addition, the signal generator 102 may be, for example,a solid state amplifier that may be computer controlled and whichcontains an internal oscillator for signal generation, but can alsoaccept an external signal source. When an external signal source isused, an external signal source port (not shown) may be used forreceiving a signal from a signal generation device that is thenamplified by the amplifier.

The transformer 104 may be any type of transformer, such as, forexample, a high-frequency step-up transformer. The operating frequencyof signal generator 102 is chosen so that in operation it places signalamplifier 102 and transformer 104 in a “resonant” mode characteristic ofa tank-circuit. This operating frequency will hereinafter be referred toas the carrier frequency, fc. In one example, the transformer 104resonates at a frequency of approximately 800 kHz or greater. Forexample, the transformer 104 has a resonant frequency of either 1 MHz or2 MHz; however in other examples, other frequencies may be used.

As described in further detail below, the signal amplifier 102 and thetransformer 104 generate a high voltage AC signal at frequency fc. Thegenerated AC signal may have any of a variety of waveforms such as, forexample, sinusoidal, square, triangular, saw tooth, etc. Because asinusoidal waveform typically produces fewer harmonics than otherwaveforms, the example generated AC signal described herein is asinusoidal waveform.

The example plasma probe 108 includes two closely spaced electrodes andis connected to the transformer 104 via a cable 106. The example cable106 is capable of carrying high voltage signals. The AC signal isprovided, via the cable 106, to the plasma probe 108 and a plasmadischarge is generated between the two electrodes. A more detaileddescription of the example plasma probe 108 is provided below.

The example plasma sensor 100, which, as illustrated, includes thesignal generator 102, the transformer 104, the cable 106 and the plasmaprobe 108, represents an RLC circuit, with the resistance, R, and thecapacitance, C, represented by the plasma probe 108 and the cable 106,and the inductance, L, represented by the transformer 104. Thus, theoverall impedance of the system is frequency dependent and has anoptimum frequency (i.e., the resonant frequency) at which the outputvoltage will be a maximum. Operating the circuit at this resonantfrequency greatly helps in achieving sufficient voltage to initiate aplasma discharge between the electrodes of the plasma probe 108.Additionally, in this example, the transformer 104 has a resistance muchgreater than the resistance of the cable 106 and the plasma probe 108.This results in the delivery of a constant current to the plasma probe108 while plasma forms between the electrodes of the probe 108regardless of variations in the resistance across the plasma probe 108during operation of the system 100. Small variations in the current mayoccur in such a constant current system. Thus, as used herein, the term“constant current” means that the current is substantially constantwhile small variations in the current may exist.

In operation, the example plasma system 100 is tuned to resonate byadjusting the frequency of the signal generator 102. When tuned, thepower drawn by the plasma probe 108 is less than approximately 5 Watts.Because the voltage of the system drops dramatically at frequenciesother than resonant frequency, the length of the cable 106, in thisexample, is kept to a minimum to avoid capacitance losses that mayreduce the resonant frequency of the system. Further, in some examples,the system may be monitored in real-time to ensure that the systemremains in resonance. This may be accomplished manually or by, forexample, hardware and/or software that monitors the system andautomatically adjusts the frequency of the signal generator 102 toensure the system remains in resonance.

The example plasma sensor 100 also, as illustrated, includes a voltageprobe 110 that measures the output of the plasma probe 108. Theillustrated example voltage probe 110 is connected to the cable 106 at a“T” junction on the cable 106 and also to a signal measurement device112. Because the voltage is very high in the illustrated example, a1000:1 high-voltage high-bandwidth probe may be used to reduce thevoltage so as not to harm the signal measurement device 112. Further, insome examples, an AM receiver may be used in place of the voltage probe108 to take advantage of the AM transmission characteristics of thegenerated plasma, as described below.

The signal measurement device 112 may be any device capable of acquiringa waveform. For example, the signal measurement device 112 may be anoscilloscope, such as a digital oscilloscope, a digital radio likedevice (e.g., a GNU radio), or other hardware, software, and/or firmwarecapable of acquiring the waveform of the signal on the cable 106.

In operation, the signal measurement device 112 acquires the waveform ofthe signal on the cable 106 and transfers the waveform to the computer114, which analyzes the acquired signal. The computer 114 may be, forexample, any standard processor based system, such as, but not limitedto, a laptop computer, a desktop computer, a workstation, a hand heldcomputer, etc. Additionally, rather than employing a separate signalmeasurement device 112 and computer 114, in some examples these devicesare combined into a single device. The combination of the signalmeasurement device 112 and the computer 114, whether as separate or in acombined device, will hereinafter be referred to as the signal analysissystem.

FIG. 2 illustrates a detailed view of the example plasma probe 108 ofFIG. 1. As illustrated, the example plasma probe 108 includes twoelectrodes 202 separated by a small air gap 204. In this example, theelectrodes 202 are very thin, such as for example, less thanapproximately 0.1 mm. In some examples, the electrodes 202 arephoto-etched by a conventional chemical milling process out of 0.457 mm(0.0018 in.) stainless hardened steel. Using a chemical milling processmay help to ensure a precise geometry of the electrodes, which increasesthe accuracy of the resulting measurements. Additionally, the electrodes202 may be manufactured from a variety of materials, such as, forexample, stainless steel, tungsten, platinum, or any other suitablematerial.

The electrodes 202 may also be coated with a dielectric material toreduce the likelihood that plasma formed between the electrodes 202during operation of the system will sputter (e.g., vacillate orotherwise fluctuate). This may also aid in improving the life of theelectrodes 202 and the accuracy of the measurements. An exampledielectric coating may be a very thin coating of, for example,approximately 1-3 microns and may be applied in an evacuated chamber asis well known to those of ordinary skill in the art. Further, thedielectric coating may be, for example, an oxide layer, such as, forexample, silicon dioxide, or any other suitable material.

Further, the electrodes 202 may be fabricated as joined pairs that canbe separated along a thinned section at the center. FIG. 3 illustrates amore detailed diagram of an electrode 202. In this example, eachelectrode is approximately 2.92 cm (1.15 in) in length, although inother examples, electrodes of longer and shorter lengths may be used.Further, the example electrodes 202 include cut-outs 304 to allowplacement of locating screws that may be used to adjust the electrodes202 and/or the gap 204.

As shown, the electrodes 202 are inserted into a carrier or fixture 206and fasteners 208 are used to secure the electrodes into place. Thefixture 206 may be, for example, a plastic fixture or any suitablecarrier and the fasteners 208 may be screw such as nylon screws or anyother suitable mechanical or chemical fasteners. In some examples, thefixture 206 may be manufactured from a 1.27 cm (0.5 in) diameter plasticrod into which a 0.51 mm (0.02 in.) slot is cut to accept the electrodes202, and the fasteners 208 pinch the sleeve to securely capture theelectrodes 202. Further in some examples, the tips of electrodes 202 areetched to about less than half the thickness of the electrodes 202 by amilling process (e.g., about less than half of 0.457 mm) to reduceaerodynamic blockage in the region of plasma discharge. Further, in someexamples, the electrodes 202 may be prepared for use by lightly sandingwith 600 grit Emory paper along the section at the tip to remove anyimperfections or contamination deposited during the chemical millingprocess, which may be accomplished by, for example, running a feelergage encased in Emory paper back and forth in the gap so that the tipsare substantially parallel to ensure an even and well-controlleddischarge. Additionally, the example plasma probe 108 may be mounted ona sting in such a way that the general form factor is similar to ahot-wire sensor. FIGS. 4A-4B provide two enlarged views of the electrodetips and the gap 204. FIG. 4A illustrates a side view and FIG. 4Billustrates an end view.

The dimensions of the electrodes 202 may be of greater concern in someexamples than in others. For example, in those examples in which changesin velocity in a flow path are measured, the electrodes 202 should havegreater aerodynamic properties. In those examples in which the sensor100 is disposed adjacent a flow path and flow disturbance such as, forexample, pressure changes are measures, the aerodynamic properties ofthe electrodes 202 may be immaterial.

In the illustrated example, the gap 204 is set small enough to preventplasma from escaping into the free-stream and “flap.” This behavior mayproduce strong sinusoidal fluctuations and higher harmonics in the ACcarrier that may saturate the signal to be measured and, thus, make itmore difficult to obtain accurate information regarding the flow.Further, in this example, the power (amplitude) of the carrier signalfor generating the plasma also is set to a low enough value to preventplasma from escaping. In addition, in this example, the power(amplitude) of the carrier signal for generating the plasma is set highenough to prevent the plasma from turning off or operating in anintermittent fashion. Preventing plasma escape and intermittent plasmaalong with maintaining precise electrodes, such as those discussedabove, helps to ensure both that the measured voltages are directlyproportional to magnitude of the flow disturbance including, forexample, the velocity of the flow, the pressure in or near the flow pathor changes thereof, and that the measurements are repeatable.

The formation of plasma between the electrodes 202 is initiated byadjusting the frequency of the signal generator 102 to bring the systeminto resonance, which maximizes the output through the transformer 104.In this example, this is done by initially setting the signal generator102 to a low input power level below the threshold necessary to startthe discharge. The power of the signal generator 102 is then increasedto the point where a glow-discharge is formed. Depending on the gap sizeand pressure, this initiation voltage may be, for example, in the rangeof about 700-1000 V_(rms) (root-mean-square) as measured by the voltageprobe 110. In some examples, after the plasma has initiated, the inputpower may be reduced slightly on the amplifier because the powerrequired to sustain the plasma is less than the power required forinitiating plasma formation. Further, as discussed above, aftergeneration of the plasma, too large of a voltage may result in theplasma escaping, thus making it more difficult to obtain accurate flowmeasurements.

In this example, once the plasma has started, the voltage across theelectrodes 202 instantly drops (e.g., to about as low as 330 V_(rms))due to the current from the flow disturbance flowing through the plasma.During operation, the voltage varies between these two limits (i.e.approximately 700-1000 V_(rms) and 330 V_(rms) in this example)depending on the magnitude of the flow disturbance including, forexample, the velocity of the flow, the pressure in or near the flow pathor changes thereof. Further, in some examples, the plasma and voltageamplitude are monitored to ensure the plasma between the electrodes 202of the plasma probe 108 is continuous. For example, if the voltage isallowed to drop too low, the plasma may sputter or become intermittent,which may make it more difficult to obtain accurate flow measurements.Further, as discussed above, if the voltage becomes too large, theplasma may escape which may also make it more difficult to obtainaccurate flow disturbance measurements. This may be accomplished by, forexample, a person or hardware, and/or software monitoring the plasma andadjusting the amplitude of signal generator 102 as necessary to ensurecontinuous plasma while also ensuring the plasma does not flap.

FIGS. 5A and 5B shows the example plasma probe 108 in proximity to acompressor rotor 502 and flush with or recessed from a turbine or rotorcasing 504. For example, the sensor and the probe 108 in particular, maybe flush-mounted to the inner wall of the casing 504 at a location justabove a blade row of the turbine. In this configuration, the sensor canmeasure pressure variations as described herein in close proximity tothe rotating blades without interfering with or being destroyed by theblade rotation.

There are other advantages that are particularly appreciated in theharsh environment of a jet engine. For example, the example, plasmasensor can perform at extreme temperature including, for example, ashigh as 1335° C. (2400° F.). In addition, in some examples the plasmaprobe may include an outer casing 508 that houses the conductors, i.e.,wires 507, 509, which are encapsulated by an insulator 510. In someexamples the insulator 510 may include protective outer metallic sheathcovering an insulating ceramic powder such as, for example, MgO. Theceramic powder 510 prevents the two wires 507, 509 from shorting duringoperation. The electrode pair 202 is formed by the two conductors 507,509, which as noted above, may be spaced about 0.15 mm (0.006 inch)apart. The junction at the end may be cut and the 507, 509 conductorsground flat using a fine file as noted above. The example probe 108 maybe placed in a casing. The casing and, thus, the sensor and probe may beany size including, for example, sized to be placed in a 12.7 mm (0.5inch) deep tapped hole sized to accept a 6-32 screw, which is roughly5.4 mm (0.137 inch) in diameter. In addition, in some examples, thecomponents may be constructed with microelectromechanical systems(MEMS).

In the example of FIG. 5A, the plug end of the outer casing 508 was usedto connect the device to the AC generator 102. In particular, one leadis connected to the high voltage, and the other is grounded. Though theplasma probe 108 may be operated with a wide variety of controlparameters, in one example, the device is operated with a 2 MHz carriersignal of roughly 350 V_(rms) at a power level of roughly 1 Watt, with amaximum of about less than 5 Watts. The frequency of the carrier signaldetermines the frequency response of the device. The practical upperlimit is determined by the mobility of the ions in the discharge, whichis on the order of about 15 MHz. The voltage at the tip may be monitoredby reading the voltage at the plug on the lead carrying the AC waveform.It is also possible to read the sensor output wirelessly. This ispossible because of the strong electromagnetic fields are emitted fromthe high-voltage lead near the electrode pair 202. This field can beeasily captured with a suitably designed antenna and offers thepotential to simplify wiring of in-situ measurements inside thecompressor section.

The example plasma sensor works on a principle of amplitude modulation.The voltage drop across the electrodes 202 is modulated by thedisturbance in the airflow that passes through the discharge region.This behavior represents an advantage of the sensor, asamplitude-modulated waveforms are resistant to signal contamination bycommon-mode noise. The details of the waveform showing the modulationcharacteristics are shown in FIG. 5B. As shown in FIG. 5B, the ACwaveform flows into the electrode pair 202. The top wire 507 shows theincoming AC waveform 515. The AC waveform forms the plasma 517 betweenthe electrodes 202 and is modulated by the incoming flow disturbance519.

FIGS. 6A-D illustrate an example effect a flow disturbance between theelectrode pair 202 has on the plasma 517 generated by an AC waveform 515for different flow disturbances 519. As shown, FIG. 6A illustrates aplasma 517 formed between electrodes 202 in the presence of no flowdisturbance 519; FIG. 6B illustrates the plasma 517 between electrodes202 in the presence of a small flow disturbance 519; FIG. 6C illustratesthe plasma 517 between electrodes 202 in the presence of a large flowdisturbance 519; and FIG. 6D illustrates the plasma 517 in the presenceof a time-dependent fluctuating flow disturbance 519. As noted above, aflow disturbance may be a velocity, a pressure or any other force,stress, interference, etc or changes thereof.

The flow disturbance 519 may cause ions to be driven out of the gap 204causing the current density to increase to maintain a constant current,which in turn forces the voltage driving the plasma to increase. Thisvoltage increase is directly measurable and may be correlated to achange in pressure and, thus, a change in flow velocity, pressure orother disturbance. Further, as illustrated, larger flow disturbances maycause plasma 517 to be deflected in gap 204. For example, plasma 517“stretches” in response to increases in mean flow disturbance 519 and“vibrates” in response to time-dependent fluctuations in the flowdisturbance 519, as shown in FIG. 6D. These time-dependent fluctuationsmay be modeled as sinusoidal signals with a disturbance frequency, fm.

It is possible to draw an analogy between the plasma 517 in gap 204 anda variable resistor. For example, the root-mean-square (r.m.s.) voltageof plasma 517 varies with the magnitude of the flow disturbance 517through the gap 204 as if a resistor was limiting the current acrosselectrodes 202. That is, this “gap resistance” varies as the magnitudeof the flow disturbance changes, thus changing the voltage drop acrossthe gap 204. Thus, as the magnitude of the flow disturbance increases onaverage, the mean voltage output from plasma probe 108 increases. Thisvoltage increase or decrease may then be measured and the averagepressure, flow velocity, etc. calculated.

In addition to determining the average magnitude of the flow disturbance519, the system also may be used to determine information regardingfluctuations in the flow disturbance 519 (e.g, periodic disturbancesetc.). For example, as discussed above with reference to FIG. 6D,periodic flow disturbances 519 may cause plasma 517 to oscillate at afrequency, fm. This is also shown in FIG. 5B. In FIG. 5B, the bottomwire 509 shows the outgoing amplitude modulated AC waveform 521, whichwas modulated by the plasma oscillation. The carrier waveformresponsible for creating the plasma is shown at frequency fc and isgenerated by an amplifier/generator. The disturbance 519 in the airflow,depicted as a periodic disturbance of frequency fm modulates the carrierto produce a classical amplitude modulated waveform with frequencycontent at fc, fc−fm, fc+fm.

As shown in FIG. 7, to recover the information in the side-bands, themodulated signal must be demodulated. When this modulated signal isanalyzed in the frequency domain, the disturbance frequency, fm willappear as two side-bands equidistant from the central peak at thecarrier frequency, fc (i.e., at fc−fm and fc+fm). The informationprovided by the ‘carrier’ at fc represents the mean-state of the flow,whereas the time-resolved unsteadiness is carried by the so-called‘side-bands’ at the difference and summation frequencies.

The demodulation may be performed by an acquisition system that isdesigned to perform the digital signal processing in an efficient manneron, for example, a host PC (see e.g., FIG. 1) so that the modulatingsignal fm can be resolved in real time. This acquisition system mayinclude software libraries and a hardware device known as the UniversalSoftware Radio Peripheral (USRP), which performs the analog-to-digitalconversion. The acquisition system provides a real-time graphical userinterface (GUI) that may be used to monitor the performance of theplasma sensor. Different signal processing blocks can be ‘wired’together in software to do various types of signal processing with theresults shown on a computer screen. This can be seen in FIG. 8, whichshows an image of an interface in which three plots are depicted: thetop represents the spectrum of the modulated signal. The time series isdemodulated in which the spectrum is converted to baseband (the middleplot). The interface shows the primary peak in the spectrum at 1 kHzalong with the higher harmonics. The last figure is the demodulated timeseries, which looks much as it would with any traditional velocity orpressure sensor.

FIG. 9 shows the example plasma sensor 100 embedded in the casing 504 ata position downstream of a leading edge 906 of the rotor 904. The flowin this example is in the direction of the arrow, X. In this example, inparticular, the position of the example plasma sensor 100 is 2.1% ofchord length downstream of the leading edge 906. The plasma sensor 100is positioned to observe maximum sensitivity to a stall inception thatmay occur between the leading edge 906 and a mid-chord 908 of the rotor.

To determine if the compressor is about to experience a stall, surge orother event, the example plasma sensor 100 measures the flowdisturbance, e.g., the pressure periodically. The pressure values aremanipulated through an algorithm to produce a correlation coefficient.The value of the coefficient is compared to a threshold value. Forexample, a decreasing coefficient may be due to the chaotic nature ofthe flow that occurs as the compressor approaches stall. The correlationis calculated from real-time statistical analysis from theover-the-rotor dynamic pressure sensors. This analysis computes acorrelation measure based on the per-rotation coherence of the pressuresignal, which is integrated over several blades. The integration time isuser defined; too large a window and features are averaged out, whiletwo small a window and the signal may be excessively noisy. In someexamples, three to five blade passages may be used in a 20-blade systemto comprise the integration window. The auto-correlation signal may besampled at least ten times the blade passage frequency. One usefulalgorithm is Equation 1, shown below.

$\begin{matrix}{{C(t)} = \frac{\sum\limits_{i = {t - {wnd}}}^{t}( {P_{i} \cdot P_{i - {shaft}}} )}{\sqrt{\sum\limits_{i = {t - {wnd}}}^{t}{P_{i}^{2} \cdot P_{i - {shaft}}^{2}}}}} & ( {{Equation}\mspace{14mu} 1} )\end{matrix}$Where t is the current sample time, C(t) is the correlation measure as afunction of time, i is the sample index, Pi is the measure pressure attime t=i, wnd is the correlation window size in number of samples, andshaft is the number of samples in one shaft revolution. The way thecorrelation is defined, the product of the pressure at a given time (Pi)and the pressure at exactly one rotation to the current rotation(Pi-shaft) are summed. The summation is performed over a window fromt-wnd to t. This value is divided by the square root of separatesummations over the individual products of Pi and Pi-shaft squared withthemselves. This measure is done is real time and may be used asfeedback to, for example, an engine controller.

The correlation measure is defined on the basis that the pressure-timetrace obtained by a sensor, e.g., the plasma sensor 100, over therotor-blades 502 will vary in its per-rotation repeatability when thecompressor is nearing stall. By the definition provided above, thecorrelation index C(t) varies from 1 to −1, but by virtue of thebehavior of the compressor the lower value is usually bound by 0. Avalue of 1 indicates a perfect repetition from one rotation to the nextof the blade passage pressure signal. Likewise, values near 1 indicatethat the system is operating away from stall the pressure signal and ismostly periodic. As the boundary of stable operation is approached, theperiodicity is disrupted and the correlation measure decreases.

As noted above, wnd represents a window over which samples are averaged.This window can be any value up to the number of samples containedwithin one shaft rotation. However, a window this large would averageout all the relevant and important transient information. Also, as notedabove, a window that is too small will have a noisy calculatedcorrelation. As noted above, in some examples the value is one thatspans between about three to five blades.

FIG. 10 is a plot showing example blade passage frequencies (BPF). Thedata used in FIG. 10 was gathered from an example plasma sensor, whichrecorded time-resolved unsteady measurements of the pressure over theblade tips. The pressure was acquired at a rate of 256 kHz, well aboveten times the blade passages frequency of less than 5 kHz. The bladepassage pressure signature is a useful indicator of the performance ofthe plasma sensor 100. No filtering other than anti-aliasing at half theNyquist frequency was performed for this data set. The plasma sensor 100is clearly able to detect the blade-passage frequency, which begins near2.5 kHz and increase to roughly 5 kHz as the compressor is spooled up tomaximum speed.

Not only is the fundamental blade passage frequency resolved, but twoadditional harmonics of this frequency can also be seen. The pressuresignal from individual blades can also be resolved, with thetime-history of five individual blades over three sequential rotationsshown in FIG. 11. The pressure signal is clearly repeatable from onerotation to the next when a particular blade is followed.

FIG. 12 shows a voltage and time plot that illustrates a compressor thatwas throttled at an intermediate speed from non-stalled operation intotransient stall and then back to a non-stalled state. The moment ofstall is clearly shown as a large perturbation of the pressure signal,with large chaotic fluctuations. In fact, the exact moment of stall isdeterminable by the occurrence of a large spike, which possiblyindicates a large-scale deflection of the plasma or even momentaryextinction of the glow-discharge. This particular example represents atransient stall starting from 59% of maximum rotational speed.

The per-rotation voltage auto-correlation is shown in FIG. 13. The topgraph represents the voltage at each revolution or fractions thereof(i.e., per time) while the lower graph shows the correlation index ateach revolution, or fraction thereof. As shown in the graph, it is clearthat the correlation index decreases significantly during the stall. Thevalue of the correlation before the stall event is near 1, and drops to0.95.

FIG. 14 illustrates that a transient short length-scale stall inceptionevent is followed by the development of rotating stall cells, which arelow-frequency disturbances in the voltage time series. These stall cellshave a duration of approximately three cells in five rotor rotations,and these rotating stall cells grow rapidly in magnitude over thefollowing rotations after the formation of the initial event and arefully developed within a few rotations after the onset of the stallinception event.

Closer inspection of FIG. 14 reveals a small dip in the correlationindex before the stall occurred. This decrease in the correlation index,though slight, is seen between 5-10 revolutions before the appearance ofthe rotating stall cells. Thus, the example plasma sensor may forecast astall and provide a warning in advance of the stall inception toimplement a stall management system to avoid a stall. Such a systemwould make it possible to operate the gas-turbine closer to the stallmargin while providing the feedback required by the control systems toimplement corrective measures before stall fully develops.

Although the teachings of the present disclosure have been illustratedin connection with certain examples, there is no intent to limit thedisclosure to such examples. On the contrary, the intention of thisapplication is to cover all modifications and examples fairly fallingwithin the scope of the appended claims either literally or under thedoctrine of equivalents.

1. A method of detecting an incipient stall of a fluid flow in an axialflow device comprising: mounting a plasma probe to an inner surface of ahousing of the axial device, the housing of the axial flow device havingan inner surface, an outer surface, a fluid inlet, a fluid outlet, and aplurality of blades rotatable within the housing; rotating the pluralityof blades within the housing to create the fluid flow between the fluidinlet and the fluid outlet; generating a voltage to form a plasma, theplasma being exposed to the fluid flow; detecting periodically at leastone characteristic of the generated plasma; measuring the characteristicof the generated plasma as the plasma is disturbed by the fluid flow;identifying a variation in the measured characteristic of the generatedplasma to predict the incipient stall of the fluid flow; and comparingthe measured characteristic to a threshold to identify the variation. 2.A method as defined in claim 1, further comprising repeatedly measuringthe characteristic of the generated plasma at a fluid flow below theincipient stall event to create the threshold.
 3. A method as defined inclaim 2, further comprising averaging the repeatedly measuredcharacteristic over a predetermined time to create the threshold.
 4. Amethod as defined in claim 1, periodically measuring the characteristicof the generated plasma for at least one individual blade of theplurality of blades.
 5. A method as defined in claim 4, furthercomprising identifying a variation in the characteristic of theindividual blade.
 6. A method as defined in claim 1, wherein theperiodically measured characteristic identifies at least one of thepressure, velocity, mass-flux, force, stress, or interference of thefluid flow.
 7. A method as defined in claim 1, wherein the measuring ofthe characteristic of the generated plasma is performed by at least oneof a voltage probe or an AM receiver.
 8. A method as defined in claim 1,wherein the axial flow device is a gas turbine.
 9. A method as definedin claim 1, wherein the processor is further configured to correlate thedetected variation in the measured characteristic to a physical changein the fluid flow.
 10. A plasma sensor for detecting a change in acharacteristic of a fluid flow in an axial flow device comprising: aplasma probe mountable in the housing of an axial flow device having aninner surface, an outer surface, a fluid inlet, a fluid outlet, and aplurality of blades rotatable within the housing to cause a fluid flowproximate the inner surface and between the fluid inlet and the fluidoutlet, the plasma probe configured to generate plasma proximate to theplurality of blades and exposed to the fluid flow; a signal generator togenerate a signal to the plasma probe to cause the plasma probe togenerate the plasma; a detection probe to periodically detect at leastone characteristic of the generated plasma; a signal measuring device tomeasure the detected characteristic; and a processor configured toidentify a variation in the detected characteristic of the generatedplasma and provide an output indicating the change, wherein theprocessor identifies the variation in the detected characteristic of thegenerated plasma by comparing the detected characteristic to a thresholdcharacteristic.
 11. A plasma sensor as defined in claim 10, wherein thewherein the signal generator is adjustable to cause the plasma be inresonance.
 12. A plasma sensor as defined in claim 10, wherein theprocessor is further configured to identify an incipient stall of theaxial flow device.
 13. A plasma sensor as defined in claim 10, whereinthe processor identifies the variation in the detected characteristic ofthe generated plasma by comparing the detected characteristic to athreshold characteristic.
 14. A plasma sensor as defined in claim 10,wherein the threshold characteristic is determined by detecting thecharacteristic of the generated plasma at a fluid flow below the ratedincipient stall of the axial flow device.
 15. A plasma sensor as definedin claim 12, wherein the threshold characteristic is associated with atleast one individual blade of the plurality of rotatable blades.
 16. Aplasma sensor as defined in claim 10, further comprising a stallmanagement system to receive the outputted indication of the incipientstall of the axial flow device and configured to modify the operation ofthe axial flow device to avoid the stall.
 17. An axial flow stallavoidance system comprising: an axial flow device having a housing and aplurality of blades rotatable within the housing to cause a fluid flowwithin the housing; a plasma probe adapted to receive an electric signalto generate a plasma, the plasma probe being mounted within the housingsuch that the generated plasma is influenced by the fluid flow; anelectric signal generator to generated the plasma; a detection probeproximate the plasma probe to periodically measure a modulated signalfrom the plasma; a signal measuring device to manipulate the modulatedsignal; a processor to identify a variation in the manipulated signaland output an indication of an incipient stall of the axial flow device,wherein the processor identifies the variation in the manipulated signalof the generated plasma by comparing the manipulated signal to athreshold; and a stall management system to receive the indication ofthe incipient stall and to influence the operation of the axial flowdevice to avoid the stall.
 18. An axial flow stall avoidance system asdefined in claim 17, wherein the electric signal generator is tunable tocause the plasma to resonate.
 19. An axial flow stall avoidance systemas defined in claim 17, wherein the stall management system isconfigured to alter the flow conditions of the plurality of blades toavoid the stall.
 20. An axial flow stall avoidance system as defined inclaim 17, wherein the plasma probe contains at least one electrode beingat least one of embedded within or flush with an inner surface of thehousing of the axial flow device.
 21. An axial flow stall avoidancesystem as defined in claim 17, wherein the plasma probe is disposedadjacent to a blade path.
 22. The plasma probe as defined in claim 21,wherein the probe is located at the mid-chord position of the bladepath.
 23. The plasma probe as defined in claim 21, wherein the probe islocated proximate to the occurrence of stall cells.